This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-169217, filed Jun. 16, 1999; and No. 2000-170145, filed Jun. 7, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a cathode-ray tube such as a color picture tube and, more particularly, to a cathode-ray tube in which a stem is welded to the end of a neck, and relates to a method of manufacturing the same.
Generally, a color cathode-ray tube has an envelope which is constituted by a glass panel having an inner surface formed with a phosphor screen, a glass funnel bonded to the panel, and a cylindrical glass neck continuously connected to the smaller end of the funnel. To the end of the neck is welded the outer peripheral portion of a flare of the stem. An electron gun is arranged in the neck. The electron gun has three cathodes lined up in line, heaters for heating the cathodes, and a plurality of electrodes sequentially adjoining the cathodes.
In the color cathode-ray tube, three electron beams emitted from the electron gun and traveling in one flat plane in line are deflected by a deflection yoke mounted on the outer surface of the funnel to scan the phosphor screen horizontally and vertically through a shadow mask, thereby displaying a color image.
Usually, the stem is comprised of a circular plate-like glass flare, an exhaust pipe extending vertically from the center of the flare outside the neck end, a plurality of stem pins extending through the flare hermetically and lined up on a circle coaxial with the center of the flare, and fillets surrounding those inner leads of the stem pins extending in the neck, which are on the flare side. The plurality of stem pins support the cathode side of the electron gun to guide a voltage applied to the heater, cathodes, and plurality of electrodes.
In an ordinary stem, of the stem pins, outer leads extending outside the neck and inner leads extending in the neck are located on a common circle. The diameter of a pin circle defined by the plurality of stem pins is determined by the outer diameter of the neck. For example, when the outer diameter of the neck is 29.1 mm, the diameter of the pin circle is 15.24 mm.
As another stem, one disclosed in Jpn. Pat. Appln. KOKAI Publication No. 58-32327 is known. In this stem, a pin circle defined by the inner leads of a plurality of stem pins is made smaller than a pin circle defined by the outer leads, and the inner and outer leads are connected to each other through Dumet wires sealed in a flare. With this arrangement, the pin circle defined by the outer leads is kept identical with that of a stem in which the outer and inner leads are located on the common circle, so that this stem can be welded to a small-diameter neck.
As an electron gun sealing method, a cullet method and a butt seal method are known.
According to the cullet method, an electron gun supported by a stem through stem pins is inserted to a predetermined position in a neck. In this process, the whole stem is inserted in the neck, and the circumference of the flare is set to oppose the inner circumferential surface of the neck. The outer peripheral portion of the flare and the opposing neck portion are welded by heating with the flame of a gas burner. Furthermore, the extra neck portion located closer to the exhaust pipe of the stem than the welded portion is sealed by burning it off with the flame of the gas burner. This method is not suitable for a small-diameter neck, since the stem must be inserted in the neck.
According to the butt seal method, a neck is cut to a predetermined size in advance. An electron gun supported by a stem through stem pins is inserted in the neck, and the end of the neck and the flare of the stem are abutted against each other or opposed to each other at a short distance. In this state, the neck end and the flare are welded by heating with the flame of a gas burner.
With the butt seal method, the stem is not inserted in the neck, so that the neck diameter can be decreased. Since the neck diameter is decreased, a deflection coil can be arranged close to electron beams, and the deflection power can be decreased. As described in Jpn. Pat. Appln. KOKAI Publication No. 58-32327, the exhaust pipe can be made thick to improve the exhaust efficiency.
As described above, the electron gun of the cathode-ray tube is arranged in the neck with its cathode side being supported by the stem, and is sealed in the neck as the neck and stem are welded to each other. This sealing structure of the electron gun is, however, sensitive to a heat shock.
Generally, the neck of a cathode-ray tube is made of glass containing 30% to 34% of PbO, which has a high electrical resistance and breakdown voltage, and having a thermal expansion coefficient xcex1(N) given by
xcex1(N)=95 to 96.5xc3x9710xe2x88x927/xc2x0C. (0xc2x0 C. to 300xc2x0 C.)
to match a thermal expansion coefficient xcex1(F)=98 to 98.5xc3x9710xe2x88x927/xc2x0C. (0xc2x0 C. to 300xc2x0 C.) of a funnel to which the neck is to be continuously connected.
The flare of the stem is made of glass having a thermal expansion coefficient xcex1(S) given by
xe2x80x83xcex1(S)=91.5xc3x9710xe2x88x927/xc2x0C. (0xc2x0 C. to 300xc2x0 C.)
to match a thermal expansion coefficient xcex1(DU)=approximately 90xc3x9710xe2x88x927/xc2x0C. (30xc2x0 C. to 400xc2x0 C.) of the Dumet wire.
Therefore, when the neck and stem are welded to each other by the butt seal method, the glass of the neck and the glass of the flare form a boundary in the lateral direction perpendicularly intersecting the tube axis (Z axis) of the envelope. When heat is applied to the welded portion, the neck expands more than the flare of the stem because of the difference between the thermal expansion coefficient xcex1(N) of the neck and the thermal expansion coefficient xcex1(S) of the flare of the stem, that is,
xcex1(N)xe2x88x92xcex1(S)=3.5 to 5.0xc3x9710xe2x88x927/xc2x0C. (0xc2x0 C. to 300xc2x0 C.)
so cracking occurs at the boundary.
The heat shock applied to the welded portion of the neck and stem is produced by heat radiated from a heater that heats cathodes while the cathode-ray tube is operated. In a color cathode-ray tube in which a high voltage of about 20 kV to 30 kV is applied to some electrodes of the electron gun, when cracking occurs in the welded portion to decrease the vacuum degree in the envelope, electric discharge occurs in the tube to make the cathode-ray tube defective.
In order to prevent cracking in the welded portion as described above, after the neck and stem are welded to each other, the welded portion must be annealed, or positional displacement between the neck and stem after the electron gun is sealed must be regulated strictly. When annealing the welded portion, to sufficiently remove stress strain generated in the welded portion, a long-time process is required, leading to a decrease in production efficiency.
Since the neck and stem are welded to each other by using a gas burner, an electron gun sealing apparatus is heated to a high temperature. To regulate the positional displacement strictly, the sealing apparatus must be checked and maintained frequently.
The present invention has been made in view of the above problems, and has as its object to provide a cathode-ray tube in which a neck and stem can be welded to each other without decreasing the production efficiency, so that the welded portion is resistant against a heat shock, and a method of manufacturing the same.
In order to achieve the above object, according to the present invention, there is provided a cathode-ray tube comprising: a glass envelope having a panel with an inner surface formed with a phosphor screen, and a cylindrical neck extending substantially coaxially with a tube axis; an electron gun arranged in the neck, for emitting an electron beam toward the phosphor screen; and a stem welded to an end of the neck. The stem has a substantially circular disk-like flare made of glass and having an outer peripheral portion welded to the end of the neck, and a plurality of stem pins attached to the flare to support the electron gun and to conduct a voltage to the electron gun. The end of the neck and the flare of the stem are welded to each other such that the glass of the flare surrounds the glass of the end of the neck from outside.
With the cathode-ray tube according to the present invention, a welded portion of the end of the neck and the flare of the stem is formed substantially annularly, and an outer peripheral edge of the welded portion is displaced from an inner peripheral edge thereof onto a panel side.
The outer and inner peripheral edges of the welded portion are preferably set at a distance from each other of not less than 1 mm along the tube axis.
According to the present invention, there is provided a method of manufacturing a cathode-ray tube comprising: a glass envelope having a panel with an inner surface formed with a phosphor screen, and a cylindrical neck extending substantially coaxially with a tube axis; an electron gun arranged in the neck to emit an electron beam toward the phosphor screen; and a stem welded to an end of the neck, the method comprising the steps of:
preparing a stem including a flare having an outer peripheral portion formed with an annular projection and a plurality of stem pins attached to the flare,
making an outer diameter of an end of the neck to be smaller than an outer diameter of the projection, and
arranging the end of the neck and the projection of the flare to oppose each other and welding the end of the neck and the projection of the flare by heating.
Furthermore, with the method of manufacturing the cathode-ray tube according to the present invention, an annular notch corresponding to the projection of the flare is formed in an outer peripheral portion of the end of the neck to reduce the outer diameter of the end of the neck to be smaller than the outer diameter of the projection.
With the cathode-ray tube manufacturing method according to the present invention, an inner circumferential surface of the projection of the flare is tilted in a tapering manner, and an outer circumferential surface of the end of the neck is tilted in a tapering manner toward a distal end thereof to correspond to the tilt of the inner circumferential surface of the projection, thereby reducing the outer diameter of the end of the neck to be smaller than the outer diameter of the projection.
According to the present invention, there is also provided a method of manufacturing a cathode-ray tube comprising: a glass envelope having a panel with an inner surface formed with a phosphor screen, and a cylindrical neck extending substantially coaxially with a tube axis; an electron gun disposed in the neck to emit an electron beam toward the phosphor screen; and a stem welded to an end of the neck, the method comprising the steps of
preparing a stem having a substantially circular disk-like flare and a plurality of stem pins attached to the flare,
arranging the stem such that an outer peripheral portion of the flare is abutted against an end face of the neck,
heating the end portion of the neck and the outer peripheral portion of the flare from outside,
fusing an outer peripheral edge of the end of the neck and an outer peripheral edge of the flare with each other, and thereafter pulling the stem along a tube axis by a predetermined distance in a direction to separate from the neck, thereby elongating the outer peripheral edge of the flare in a direction of the tube axis, and
further heating the end portion of the neck and the outer peripheral portion of the flare from outside, thereby welding the end of the neck and an entire of the outer peripheral portion of the flare.
According to the cathode-ray tube having the above arrangement and the manufacturing method therefor, since the end of the neck and the stem are welded to each other such that the outer peripheral portion of the flare surrounds the end of the neck from outside, the stress generated in the welded portion is a compression stress, and consequently the cathode-ray tube becomes resistant against a heat shock, and cracking in the welded portion can be prevented. In addition, it suffices if annealing after welding and regulation for positional displacement between the neck and stem are performed to the same level as in the conventional case. As a result, a cathode-ray tube can be manufactured without degrading the production efficiency.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.